It is the Bank Zachodni WBK S.A. in Poland belonging to the Santander Group.
As to why it is chosen by the 2 largest Bitcoin exchanges and many more i can only speculate, because its certainly not the fees nor their services. There are many banks in Poland offering much less bureaucracy and better customer support.
It might be the loyalty, since MtGox took over an account there, while prolly being rejected by most banks back in the days.
It might be because its impossible to chargeback from them.
But most probably its a fact known just to friends of DPR.